In the mechanisms of stomatal opening, the transports of osmotic materials between the guard cell cytoplasm and vacuole have not been studied much. There were also important lacks of understanding about tonoplast transport proteins and channels. Tonoplast has been found to have many types of channels related to K+ transport, among which are inward-K+ channels/FV, outward-K+ channels/FV, outward-TPK/VK channels and TPC1/SV channels. The two H+ transport enzymes in tonoplast, H+-ATPase and H+-PPase, transport H+ from the cytoplasm to vacuole very actively. They serve to create an ideal pH condition between vacuole and cytoplasm to facilitate the many metabolisms in the cell. The cytosolic K+ cannot easily enter the vacuole to fill the charge balances, because vacuole is too full of positive charges. Therefore, in order to increase the osmotic pressure of the guard cell vacuole, it is necessary to transport solute that can replace K+. Tonoplast contains sucrose-H+ antiports, an active transport protein that can transport cytoplasmic sucrose to vacuole. Although various solutes including K+ are required for stomata to open, sucrose is believed to be the most important substance that can increase the vacuole’s osmotic pressure.
